Polyamide fluidized-bed-coating powder for thin-layer fluidized-bed coating

ABSTRACT

One embodiment of the present invention provides a polyamide powder, which includes polyamide particles having a median grain size d 50 of from 20 to 90 μm, a content of fines &lt;5 μm of below 1% by weight, and at least 75% by weight of spherical particles in which all three spatial axes x, y and z of the individual particles have the same dimension to within ±10%. Other embodiments of the invention provide a process for making and using the powder, and articles coated articles obtained thereby.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polyamide fluidized-bed coatingpowders, methods of making, and methods of use.

2. Discussion of the Background

Polyamide powders based on nylon-11 and nylon-12 and specificallydeveloped for the fluidized-bed coating process generally have a mediangrain diameter d 50 to DIN EN ISO 4610 of from 95 to 120 μm. Theygenerally have a bulk density to DIN 53 466 of from 400 to 600 g/l.

The powders are supplied as precipitated or ground powders made from thecorresponding granular polyamide. Ground powders are produced bygrinding in a mill, and precipitated powders are produced by dissolvingthe granular material in a solvent and precipitating, as in DE 29 06 647B 1. The latter process gives a polyamide powder with round grain shape,which is a consequence of the precipitation process. Unlike theprecipitated powder, the ground powder has a sharp-edged grain.

Examples of precipitated nylon-12 powders are VESTOSINT® 1101, 1111,1121, 1141, and 1161 (Degussa AG), and examples of ground nylon-11powders for fluidized-bed coating are Rilsan® T 7260 gray and/7050 white(Atofina).

In the fluidized-bed coating process, hot metal parts are immersed intoa pan with fluidizing polymer powder. The powder sinters on the hotmetal surface and coalesces to give a homogeneous coating. Aprecondition for this is that the metal surface has a temperature abovethe melting point of the polymer powder.

Typically, a polyamide coating in a fluidized-bed coating process has alayer thickness of from 200 to 500 μm, or in specific instances even upto about 1000 μm. Layers thinner than about 200 μm are impossible orvery difficult to produce by the fluidized-bed coating process usingconventional fluidized-bed coating powders.

In coating technology, experience has led to acceptance that when usingthe polyamide fluidized-bed-coating powders commonly encountered in themarket the thickness of a smooth coherent film has to be at least 2×d 50of the powder. Since commercially available fluidized-bed-coatingpowders have a d 50 value of from 95 to 120 μm, the resultant lowerlimit for coating is about 200 μm.

The difficulties with lower film thicknesses are a consequence ofimmersion time and object temperature. On the one hand, the hot objectcan be immersed only briefly in order that there is no excessive growthof layer thickness on the surface, but on the other hand the powderneeds a certain minimum immersion time in the bath if all of the cornersand edges of the particular object are to be covered so that there areno uncoated defective areas.

For thin coatings of from 50 to 200 μm, therefore, use is frequentlymade of processes other than fluidized-bed coating, examples being ESspraying, high-temperature spraying, tribo spraying, and minicoating.

Minicoating is the most similar to fluidized-bed coating. Here, hotmetal parts fall into a polyamide powder bed which, however, does notbecome fluidized. The metal parts pick up powder for as long as there isenough energy to melt the polymer. Initially a rough coating forms,since the component does not possess sufficient heat to make theslightly sintered powder coalesce to give a homogenous layer. If asmooth homogeneous surface is desired, this can be achieved bypost-heating in an oven or irradiating with a heat source.

This process has been used especially for small and light metal parts,for example for corsetry clips. Examples of typical minicoating powdersare VESTOSINT® 1164, 1174, and 2157 (Degussa) and Rilsan® 1452 MAC(Atofina). The median grain size d 50 of these powders is typically from20 to 70 μm.

The bulk density of minicoating powders is mostly somewhat lower thanfor fluidized-bed coating powders. It is generally above 300-350 g/l.

Typical minicoating powders have markedly poorer fluidizing propertiesthan specific fluidized-bed-coating powders, since their bulk density islower and their grains are finer. Commercially available fine powdersand minicoating powders have limited applicability in the typicalfluidized-bed coating process.

However, there has recently been increased demand for polyamide powdersintended to achieve thin layers in the range from 50 to 200 μm in thefluidized-bed coating process.

An example of an application of this type is the chromium-free coatingof metal pipes for corrosion protection in the automotive industry, forexample as described in U.S. Pat. No. 6,276,400 B1. For continuous pipecoating as in U.S. Pat. No. 6,276,400 B1 the traditional minicoatingprocess is unsuitable. The process requires a fluidized-bed coating panthrough which the pipe is continuously drawn. In this process there islocal heating of the pipe to a temperature above the melting point ofthe polymer, preferably by induction.

The coating of pipes of this type for the automotive industrynecessitates stringent requirements for layer thickness and coatinghomogeneity. On the one hand, the layer thickness should be as small aspossible to save weight, but on the other hand the polyamide layerserves as corrosion protection on a component important for safety(brake piping, fuel piping) and therefore has to have absolutely nodefects or variations in mechanical properties.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to develop apolyamide fluidized-bed-coating powder useful for thin-layerfluidized-bed coating which permits the production of very homogeneousthin coatings.

This object and others has now been attained with the present invention,the first embodiment of which provides a polyamide powder, whichincludes polyamide particles having:

-   -   a median grain size d 50 of from 20 to 90 μm,    -   a content of fines <5 μm of below 1% by weight, and    -   at least 75% by weight of spherical particles in which all three        spatial axes x, y and z of the individual particles have the        same dimension to within ±10%. Other embodiments of the        invention provide a process for making and using the powder, and        articles coated articles obtained thereby.

Another embodiment of the invention provides a process, which includes:

-   -   screening a polyamide powder to remove particles having a size        of >125 μm to obtain a screened powder;    -   mechanically post-treating the screened powder to round off        corners and edges, to obtain a post-treated powder; and    -   removing a portion of fines having a size of <5 μm from the        post-treated powder, to obtain the above-mentioned polyamide        powder.

Another embodiment of the invention provides a coated surface, preparedby a process which includes, in a fluidized-bed coating process,fluidizing the above-mentioned polyamide powder, contacting thefluidized powder with a metal surface, and forming a polyamide coatingon the metal surface.

Another embodiment of the invention provides a process for coating asurface, which includes, in a fluidized-bed coating process, fluidizingthe above-mentioned polyamide powder, contacting the fluidized powderwith a metal surface, and forming a polyamide coating on the metalsurface.

BRIEF DESCRIPTION OF THE FIGURES

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein anautomatic system for external coating of pipes by fluidized-bed coatingis shown, having the following parts.

-   -   1) the pre-treatment system for cleaning the pipes;    -   2) the primer (adhesion promoter) pan for applying the adhesion        promoter between steel surface and polymer layer (spray system        or immersion system);    -   3) the medium-frequency induction coil 1 for baking the primer        and, if a solvent-containing primer is used, for evaporating the        solvent;    -   4) the radial fan for faster dissipation of the evaporated        solvent;    -   5) the medium-frequency induction coil 2 for preheating the        pipe;    -   6) the fluidized-bed coating pan with integrated        medium-frequency induction coil 3 for applying the coating        material;    -   7) the internals in the fluidized-bed coating pan, composed of        air-flush system above the pipe to eliminate powder        accumulations and of metal flow guide panels below the pipe to        eliminate powder shortages and resultant pores on the underside        of the pipe;    -   8) the medium-frequency induction coil 4 for smoothing the        incompletely molten polyamide layer;    -   9) the melting section, needed for thorough melting and thus        smoothing of the adherent polyamide deposit after the pipe        emerges from the medium-frequency induction coil 4;    -   10) the air-flush system for preliminary cooling of the pipe        surface;    -   11) the water-based cooling system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various other objects, features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood from the following detailed description of the preferredembodiments of the invention.

Surprisingly, it has now been found that use of the polyamide powder ofthe invention can produce very homogeneous layer thicknesses of from 50to 200 μm by the traditional fluidized-bed coating process. This rangeincludes 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 120, 130, 140, 150,160, 170, 180, 190 and 195 μm.

Unlike commercially available polyamide fluidized-bed-coating powders,the polyamide powder of the invention has fine particles, and itsparticle size is similar to that of a minicoating powder. The mediangrain diameter d 50 of the powder of the invention is from 20 to 90 μm.The upper grain size limit is 125 μm. The comparatively small particlesize permits production of thin and uniform coatings below 200 μm. Theabove range for the median grain diameter d 50 of the powder of theinvention includes 25, 30, 40, 45, 50, 55, 60, 65, 70, 75, 80 and 85 μm.The above range for the upper grain size limit includes 120, 115, 110,100, 90, 80, 70, 60, and 50 μm.

In order that the powder nevertheless has good fluidizing properties,the powder of the invention has specific characteristics. Unlikeconventional minicoating powders and fine powders for the coatingindustry, the powder of the invention is free from extreme fines, the <5μm content being well below 1%. This range includes ≦0.9, 0.7, 0.5, 0.3,0.1, 0.09, 0.07, 0.05, 0.03, 0.01, 0.009, 0.007, 0.005, 0.003. and0.001%.

Another characteristic feature of the powder of the invention is theparticularly round grain shape, which achieves a significant improvementin fluidizing properties.

The powder of the invention has a content of above 75% of substantiallyspherical particles in which all three spatial axes x, y and z of thegrain have the same dimension to within ±10%. This range includes ±9, 8,7, 6, 5, 4, 3, 2, and 1%.

Preferably, the dimension is measured as the diameter of the particle.This can be supported by REM.

Preferably, the percent contents are measured with respect to the weightof the powder unless otherwise stated.

The powder of the invention preferably has a content of above 80% ofsubstantially spherical particles in which all three spatial axes x, yand z of the grain have the same dimension to within ±10%. This rangeincludes ≧81, 83, 85, 87, 90, 93, 95, 97 and 99%.

The particle size distribution is determined by laser diffraction(Malvern) to DIN EN ISO 4610 (the entire contents of which being herebyincorporated by reference), and the grain shape is determined via modernimage evaluation systems and scanning electron micrographs. The powdersof the invention are preferably produced from a precipitated powder asin DE 29 06 647 B1, the entire contents of which being incorporatedherein by reference. The starting material provided by this process isintrinsically rounder than a conventional ground powder. The particularfeature of the powder of the invention is the mechanical post-treatmentin which corners and edges present are rounded off as required,typically in slow-running mills or specific impact equipment whichavoids complete comminution/break-up of the particles but breaks off, orrounds off, corners and edges.

Low-viscosity polyamide powders are preferably used in order that thepowder coalesces particularly readily and uniformly. The relativesolution viscosity (η_(re1)) of the powder of the invention to EN ISO307 (the entire contents of which being incorporated herein byreference) is from 1.30 to 1.65, preferably from 1.40 to 1.63. Theseranges include 1.31, 1.33, 1.35, 1.37, 1.39, 1.41, 1.43, 1.45, 1.47,1.49, 1.50, 1.51, 1.53, 1.57, 1.59, 1.60 and 1.61.

In order to improve the flow behavior when the thin-layer fluidized-bedpowder melts, regulators containing amino end groups and/or containingcarboxy groups can be added to slow the post-condensation. These may bedicarboxylic acids or diamines, for example. Depending on the regulator,the preferred ratio between amino end groups and carboxy end groups is≦1:3, or ≧3:1. These ranges include 1:4, 1:5, 1:6, 1:7, 1:9 and 1:10;and 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 and 10:1.

The flowability or fluidizability of the powder of the invention may beoptimized via additives. These may be finely divided inorganicadditives. Commercially available examples are titanium dioxides,aluminum oxides, and fine-particle silicas.

The present invention desirably allows for the chromate-free coating ofpipes by fluidized-bed coating with the present polyamide powder.

The polyamide powders of the invention are particularly used in thethin-layer fluidized-bed coating process. A typical system for coatingmetal pipes is described below.

The polyamide powders of the invention may be applied using any of theknown fluidized-bed coating processes. The system described below isparticularly suitable for chromate-free coating by the fluidized-bedcoating process. The system is automatic and serves for external coatingof pipes by fluidized-bed coating. It is composed of the followingparts:

-   -   1) the pre-treatment system for cleaning the pipes, which are        mostly delivered greased;    -   2) the primer (adhesion promoter) pan for applying the adhesion        promoter between steel surface and polymer layer (spray system        or immersion system);    -   3) the medium-frequency induction coil 1 for baking the primer        and, if a solvent-containing primer is used, for evaporating the        solvent;    -   4) the radial fan for faster dissipation of the evaporated        solvent;    -   5) the medium-frequency induction coil 2 for preheating the        pipe;    -   6) the fluidized-bed coating pan with integrated        medium-frequency induction coil 3 for applying the coating        material. The dielectric dissipation factor of the coating        material is too low for it to become heated, whereas the        preheated steel pipe passing through the system becomes heated        very rapidly to the desired temperature. The decisive factors        controlling layer thickness during fluidized-bed coating are        preheat temperature and immersion time. In the case where a pipe        is passing through the system this means that the layer        thickness can be controlled via the generator power and the        advance rate of the pipe. Both of these can be controlled        independently of one another from the control desk;    -   7) the internals in the fluidized-bed coating pan, composed of        air-flush system above the pipe to eliminate powder        accumulations and of metal flow guide panels below the pipe to        eliminate powder shortages and resultant pores on the underside        of the pipe. Uniform layer thickness both radially and axially        can be ensured only by using the specific internals;    -   8) the medium-frequency induction coil 4 for smoothing the        incompletely molten polyamide layer;    -   9) the melting section, needed for thorough melting and thus        smoothing of the adherent polyamide deposit after the pipe        emerges from the medium-frequency induction coil 4. During        passage through the system the layer is still hot and soft and        can therefore easily be damaged. Passing of the pipe over        rollers is therefore not permissible in this phase;    -   10) the air-flush system for preliminary cooling of the pipe        surface. This controls the pipe surface temperature to below the        melting point of the polyamide;    -   11) the water-based cooling system. The pipe runs into a water        trough in which the layer further cools and hardens, and passage        over rollers is therefore again possible here.

The induction coils of 5, 6, and 8 are used as required by the desiredlayer thickness. The possibilities here for use of the induction coilsare:

-   5 and 8,-   5 and 6,-   5, 6, and 8,-   6,-   6 and 8.

The pipes are heated by medium-frequency induction. Using the process,it is possible for pipe sections of any desired length to be coupledtogether to give a continuous line and to be coated externally withpolymer powder by a process involving horizontal passage through thesystem. The homogeneous layer thickness distribution of the externalpipe coating is from 120 to 180 μm, preferably 150 μm. A commerciallyavailable adhesion promoter (e.g. VESTOSINT adhesion promoter WS 5) isapplied to the pipe surface. The layer thickness of the primer afterair-drying is from 5 to 8 μm, which range includes 5.1, 5.3, 5.5, 5.7,5.9, 6.1, 6.3, 6.7, 6.9, 7.3, 7.5, 7.7 and 7.9 μm. The solids content ofthe adhesion promoter is about 8%.

The pipes produced by the process of the invention are particularlysuitable as hydraulic piping or brake piping, e.g. for the automotiveindustry.

Other preferred embodiments are listed below in A-G:

-   -   A. A polyamide powder for coating metal surfaces by the        fluidized-bed coating process, wherein        -   the powder has a median grain size d 50 of from 20 to 90 μm,        -   the content of fines <5 μm is below 1%, and        -   the powder has at least 75% of spherical particles in which            all three spatial axes x, y and z of the grain have the same            dimension to within ±10%.    -   B. The polyamide powder as recited in A, wherein the powder has        at least 80% of spherical particles in which all three spatial        axes x, y and z of the grain have the same dimension to within        +10%.    -   C. The polyamide powder as recited in any of A-B, which includes        a polyamide selected from the group including nylon-11 and        nylon-12.    -   D. The polyamide powder as recited in any of A-C, which has an        η_(re1) of from 1.30 to 1.65.    -   E. The polyamide powder as recited in any of A-D, which has an        η_(re1) of from 1.40 to 1.63.    -   F. The polyamide powder as recited in any of A-E which has been        regulated and has a ratio of ≧3:1 or ≦1:3 between amino end        groups and carboxy end groups.    -   G. A fluidized-bed coating based on the powder as recited in any        of A-F, wherein the polyamide layer thickness is from 50 to 200        μm.

EXAMPLES

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

Example 1

Preparation of the powder A of the invention:

A granular nylon-12 regulated using dicarboxylic acids and having anamino end group content of 40 mmol/l and a carboxy end group content of180 μmol and an η_(re1) of 1.55 is precipitated as in DE 29 06 647 fromhot alcohol. The precipitated powder has a median grain size d50 of 61μm and has a proportion of about 70% of spherical particles in which allthree spatial axes x, y and z of the grain have the same dimension towithin ±10%.

The resultant crude powder is approximately comparable with a typicallycommercially available fine polyamide powder, such as VESTOSINT 2157.

In a further operation, the crude powder is first freed from coarseparticles >125 μm through a screen, and is post-treated for 10 minutesin an impact pan mill (Hybridizer, NARA). The resultant treated powderis then freed from extreme fines in a pneumatic classifier. Theproportion <5 μm is 0.1%.

The resultant powder has a median grain size d 50 of 52 μm and has aproportion of 84% of spherical particles in which all three spatial axesx, y and z of the grain have the same dimension to within ±10%. Thepowder has a bulk density of 380 g/l. The powder is treated with 1.0part of flame black and 0.05 part, based on 100 parts of polyamidepowder, of a fine-particle silica (Aerosil 200).

Coating Trials

The powder of the invention from example 1 is processed on the trialsystem described above for coating metal pipes with the aid of inductionheating. For comparison, a number of commercially available polyamidepowders (Degussa AG) were used. These are VESTOSINT 1111 black, acommercially available fluidized-bed coating powder, VESTOSINT 1174white, a minicoating powder comprising titanium dioxide, and VESTOSINT2157 black, a fine powder used, inter alia, in coil coatings. Theresults are given in table 1.

All of the pipes were pretreated with VESTOSINT adhesion promoter WS 5(Degussa), layer thickness about 5 μm. TABLE 1 Max. Propor- Pipe radialtion of layer layer spherical thickness thickness Dust- Fluidiza-Coating trials d 50 <5% particles achieved difference ing tion Product(μm) (%) (%) (μm) (μm) (sec) (grade) Polyamide 52 0.1 84 120 <5 <5 1-2powder A VESTOSINT 2157 57 0.5 ˜70 120 10 10 3 VESTOSINT 1111 100 0.1˜65 200 <5 <5 1 VESTOSINT 1174 40 8 ˜70 130 20 >15 5

The polyamide powder of the invention gave a very homogeneous coating onthe metal pipe, the quality of the coating reaching that of atraditional fluidized-bed-coating powder. In terms of dusting andfluidization, the powder exhibits comparably good processing properties.The polyamide powder of the invention can achieve desired layerthicknesses below 200 μm. Satisfactory layer thicknesses of 120 μm couldbe achieved in the trial reproducibly, without defects.

In contrast, the only layer thicknesses which could be achieved incomparable quality using commercially available fluidized-bed-coatingpowders were 200 μm and above.

Conventional, commercially available minicoating powders andconventional, commercially available fine powders exhibit markedlypoorer fluidization properties in comparison, and more dusting at thefluidizing pan.

Although coherent layers of from 120 to 130 μm could be achieved on thetest system, these exhibit markedly greater coating inhomogeneity,attributed mainly to the poorer fluidizing behavior. The radial layerthickness difference is determined by evaluating photographs of crosssections.

This application is based on German patent application DE 102 33 344.0,filed Jul. 23, 2002, the entire contents of which are herebyincorporated by reference, the same as if set forth at length.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A polyamide powder, comprising polyamide particles having a mediangrain size d 50 of from 20 to 90 μm, a content of fines <5 μm of below1% by weight, and at least 75% by weight of spherical particles in whichall three spatial axes x, y and z of the individual particles have thesame dimension to within ±10%.
 2. The polyamide powder as claimed inclaim 1, wherein at least 80% by weight of spherical particles in whichall three spatial axes x, y and z of the individual particles have thesame dimension to within ±10%.
 3. The polyamide powder as claimed inclaim 1, wherein the polyamide is selected from the group consisting ofnylon-11 and nylon-12.
 4. The polyamide powder as claimed in claim 1,which has an η_(re1) of from 1.30 to 1.65.
 5. The polyamide powder asclaimed in claim 1, which has an η_(re1) of from 1.40 to 1.63.
 6. Thepolyamide powder as claimed in claim 1, wherein the polyamide furthercomprises one or more regulators having amino end groups and carboxy endgroups, and wherein a ratio of the amino end groups to the carboxy endgroups is ≧3:1 or ≦1:3.
 7. The polyamide powder as claimed in claim 6,wherein the regulators are selected from the group consisting ofdicarboxylic acid, diamine, and combinations therof.
 8. The polyamidepowder as claimed in claim 1, wherein the powder further comprises atleast one selected from the group consisting of titanium dioxide,aluminum oxide, and silica.
 9. The polyamide powder as claimed in claim1, wherein the powder has an upper grain size limit of 125 μm.
 10. Aprocess, comprising: screening a polyamide powder to remove particleshaving a size of >125 μm to obtain a screened powder; mechanicallypost-treating the screened powder to round off corners and edges, toobtain a post-treated powder; and removing a portion of fines having asize of <5 μm from the post-treated powder, to obtain the polyamidepowder product as claimed in claim
 1. 11. The process as claimed inclaim 10, wherein said post-treating comprises rounding off the cornersand edges in a mill, an impact device, or both.
 12. The process asclaimed in claim 10, wherein the portion of fines are removed in apneumatic classifier.
 13. The process as claimed in claim 10, furthercomprising, in a fluidized-bed coating process, fluidizing saidpolyamide powder product, and contacting the fluidized powder with ametal surface.
 14. A coated surface, prepared by a process comprising,in a fluidized-bed coating process, fluidizing said polyamide powderproduct, contacting the fluidized powder with a metal surface, andforming a polyamide coating on said metal surface.
 15. The coatedsurface as claimed in claim 14, wherein said coating comprises apolyamide layer having a thickness of 50 to 200 μm.
 16. A process forcoating a surface, comprising, in a fluidized-bed coating process,fluidizing the polyamide powder as claimed in claim 1, contacting thefluidized powder with a metal surface, and forming a polyamide coatingon said metal surface.
 17. The process as claimed in claim 16, whereinsaid coating has a thickness of 50 to 200 μm.